Fungi exist primarily as filamentous dikaryotic organisms.

As part of their life cycle, fungi produce spores. In this electron micrograph
of a mushroom gill, the four spores produced by meiosis (seen in the center
of this picture) are carried on a clublike sporangium (visible to the
left and right). From these spores, haploid hyphae grow and ramify,
and may give rise to asexual sporangia, special hyphae which produce spores
without meiosis.

The sexual phase is begun when haploid hyphae from two
different fungal organisms meet and fuse. When this occurs, the cytoplasm
from the two cells fuses, but the nuclei remain separate and distinct. The
single hypha produced by fusion typically has two nuclei per "cell", and is
known as a dikaryon, meaning "two nuclei". The dikaryon may live
and grow for years, and some are thought to be many centuries old. Eventually, the dikaryon forms sexual sporangia in which the nuclei fuse
into one, which then undergoes meiosis to form haploid spores, and the cycle
is repeated.

Some fungi, especially the
chytrids and zygomycetes,
have a life cycle more like that found in many
protists. The organism is haploid, and
has no diploid phase, except for the sexual sporangium. A number of fungi have
lost the capacity for sexual reproduction, and reproduce by asexual spores or by
vegetative growth only. These fungi are referred to as Fungi Imperfecti,
and include, among other members, the athlete's foot and the fungus in
bleu cheese. Other fungi, such as the yeasts, primarily reproduce through
asexual fission, or by fragmentation -- breaking apart,
with each of the pieces growing into a new organism.

Fungi are heterotrophic.

Fungi are not able to ingest their food like animals do, nor can they
manufacture their own food the way plants do. Instead, fungi feed by
absorption of nutrients from the environment around them. They
accomplish this by growing through and within the substrate on
which they are feeding. Numerous hyphae network through the wood,
cheese, soil, or flesh from which they are growing. The hyphae secrete
digestive enzymes which break down the substrate, making it easier for the
fungus to absorb the nutrients which the substrate contains.

This filamentous growth means that the fungus is in intimate contact with its
surroundings; it has a very large surface area compared to its volume. While
this makes diffusion of nutrients into the hyphae easier, it also makes the
fungus susceptible to dessication and ion imbalance. But usually this is not a
problem, since the fungus is growing within a moist substrate.

Most fungi are saprophytes, feeding on dead or decaying material.
This helps to remove leaf litter and other debris that would otherwise
accumulate on the ground. Nutrients absorbed by the fungus then become
available for other organisms which may eat fungi. A very few fungi
actively capture prey, such as Arthrobotrys which snares nematodes
on which it feeds. Many fungi are parastitic, feeding on living
organisms without killing them. Ergot, corn smut, Dutch elm disease, and
ringworm are all diseases caused by parasitic fungi.

Mycorrhizae are a symbiotic relationship between fungi and plants.

Most plants
rely on a symbiotic fungus to aid them in acquiring water and
nutrients from the soil. The specialized roots which the plants grow and the
fungus which inhabits them are together known as mycorrhizae, or
"fungal roots". The fungus, with its large surface area, is able to soak up
water and nutrients over a large area and provide them to the plant. In
return, the plant provides energy-rich sugars manufactured through
photosynthesis. Examples of mycorrhizal fungi include truffles and
Auricularia, the mushroom which flavors sweet-and-sour soup.

In some cases, such as the vanilla
orchid
and many other orchids, the young plant cannot establish
itself at all without the aid of its fungal partner. In liverworts, mosses,
lycophytes, ferns, conifers, and
flowering
plants, fungi form a symbiotic
relationship with the plant. Because mycorrhizal associations are found in
so many plants, it is thought that they may have been an essential element
in the transition of plants onto the land.

More information on one ecologically and economically important group
of fungi, the Uredinales or rust fungi, is available through the
Arthur Herbarium at
Purdue University.